Thermal dye transfer systems have been used to obtain prints electronically-generated by color video cameras.
Such prints can be produced by first subjecting an electronic picture to color separation with color filters. The respective color-separated images are converted to electrical signals and then processed to produce cyan, magenta and yellow electrical signals which are transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiver element, with both elements being between a thermal printing head and a platen roller. The thermal printing head has many heating elements that are heated up sequentially in response to the cyan, magenta and yellow signals to transfer donor sheet dye to the receiver sheet. The process is repeated for the other two colors, and a color hard copy is obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal dye transfer processes have also utilized a laser diode instead of a thermal printing head. This type of imaging process is also known as laser thermal dye transfer ("LTDT"). In such systems, the dye-donor element sheet also contains a near-infrared radiation absorbing material. The dye-donor element is irradiated with a near-infrared laser diode, and the near-infrared absorbing material converts the light energy to thermal energy. As a result, the dye is heated to volatilization and transferred to the receiver. The radiation absorbing material may be present in a layer beneath the dye or admixed with the dye. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image, so that each dye volatilizes only where it is required on the receiver to reconstruct the original image. Further details of this process are found in GB 2,083,726A, the disclosure of which is hereby incorporated by reference.
In GB 2,083,726A, carbon is disclosed as the absorbing material for use in a LTDT system. However, carbon tends to clump when coated which may degrade the transferred dye image. Additionally, carbon may transfer to the receiver by sticking or ablation, producing a mottled or desaturated color image.
Other types of non-carbon, infrared absorbing materials have also been disclosed for laser systems. However, most of these materials also absorb light in the visible region of the electromagnetic spectrum. If the near-infrared absorbing sensitizer absorbs visible light and also migrates with the desired colorants upon heating, then the unwanted visible light absorptions will change the hue and/or color density of the resultant image. U.S. Pat. No. 4,912,083 to Chapman et al. discloses an example of such an absorbing material.
Because most of the available near-infrared absorbing sensitizers are heat-transferable and absorb visible radiation, there is a need for compositions that both absorb strongly in the near-infrared region of the electromagnetic spectrum and are also "bleachable". Bleachable near-infrared absorbers are those compounds whose visible light absorption may be significantly reduced or, preferably, eliminated.
It is known that certain dyes, when combined with certain "acid-photogenerating" compounds, will bleach when exposed to appropriate activating radiation. For instance, in U.S. Pat. No. 4,769,459 to Patel, et al. the combination of a bleachable dye in reactive association with an iodonium ion is disclosed as the image-forming component in an oxidative imaging process.
U.S. Pat. No. 4,632,895 to Patel discloses the combination of a bleachable dye and an iodonium ion as the image-forming component of a diffusion/sublimation imaging system. Patel discloses the use of a variety of exposure sources to effect bleaching for the purpose of image creation. However, in all embodiments, Patel requires an additional step to actually transfer the image. In Patel, the image is first formed, by bleaching, on the dye-donor element and then heated or diffused with a liquid medium onto the image-receiving layer.